Method and device for detecting leaks in respiratory gas supply systems

ABSTRACT

The invention relates to a method and a device for detecting leaks in respiratory gas supply systems. Both the pressure and the volume flow of the respiratory gas are detected and the relevant values are supplied to an evaluation device. The evaluation device is used to calculate both the respiratory parameter resistance and compliance and the leak for at least two successive breathing cycles. At least one control parameter with different signal amplitudes is pre-determined for the successive breathing cycles. The leak resistance is determined from the resulting differential sequences of pressure and flow for the successive breathing cycles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method for detecting leaks in ventilationsystems, in which both the pressure of the respiratory gas and thevolume flow of the respiratory gas are detected and fed into anevaluation unit.

The invention also concerns a device for detecting leaks in ventilators,which has both a system for detecting the pressure of the respiratorygas and a system for detecting the volume flow of the respiratory gas,and in which the detection systems are connected to an evaluation unit.

2. Description of the Related Art

In the performance of ventilation, various control methods for theventilators are used. The devices are controlled mainly by means ofautomatic pressure control, so that a well-defined flow volume perrespiratory cycle is not determined. Control of the device on the basisof volume control of the flow volume associated with a given respiratorycycle is also possible in principle but so far has been fraught withproblems, because leaks can occur in the immediate area of theventilator, in the area of the ventilation mask, and in the area of theconnecting hose between the ventilator and the ventilation mask. Leakscan also occur in the area of contact between the ventilation mask andthe patient's face. Total leakage losses from the complete ventilationsystem comprising these individual components are often on the order of50% of the flow volume produced by the ventilator.

The previously known methods and devices for determining leakage lossesare not yet sufficiently suitable for integration in an automaticcontrol system for the device.

SUMMARY OF THE INVENTION

Therefore, the objective of the present invention is to improve a methodof the aforementioned type in such a way that improved control of thedevice is supported.

In accordance with the invention, this objective is achieved byrecording the respiratory parameters of pressure and flow for at leasttwo successive respiratory cycles with the evaluation unit, bypresetting at least one control parameter with different signalamplitudes for the successive respiratory cycles, and by determiningresistance, compliance, and leak resistance from the resultingdifferential curves of pressure and flow for these respiratory cycles.

A further objective of the present invention is to design a device ofthe aforementioned type in such a way that volume-based control of thedevice is supported.

In accordance with the invention, this objective is achieved bydesigning the evaluation unit for determining the respiratory quantitiesof pressure and flow, by providing a storage device for at least onepair of value sequences of pressure and flow for a respiratory cycle,and by making it possible to generate at least one differential sequencefor determining differential curves of compliance and resistance for atleast two successive respiratory cycles.

On the basis of the determined leakage, it is possible to engineer theventilator to supply an increased flow volume in such a way that,allowing for the leakage losses, a precisely defined useful volume flowis supplied. By taking the difference of the measured sequences ofvalues of pressure and flow in two successive respiratory cycles, suchthat the selected control parameter has different control parameters inthe respiratory cycles, it is possible to eliminate the effect ofunknown quantities.

Direct leak detection for helping to achieve exact volume flow controlis assisted by performing the computation for at least two immediatelysuccessive respiratory cycles.

To reduce the processing work, it is also possible to perform thecomputation for at least two respiratory cycles that are separated by atleast one other respiratory cycle.

In accordance with a modification, different pressure levels are presetfor the successive inspirations.

In particular, it is intended that the first pressure level be selectedhigher than the second pressure level.

In addition, it is also possible for the first pressure level to beselected lower than the second pressure level.

In accordance with another modification, it is also possible fordifferent volume flows to be preset for the successive respiratorycycles.

In this modification as well, it is intended, in particular, that thefirst volume flow be preset higher than the second volume flow.

Moreover, it is also possible for the first volume flow to be presetlower than the second volume flow.

To guarantee that a preset desired value be maintained correctly onaverage, it is proposed that a large number of respiratory cycles, eachwith a varied control parameter, be carried out in such a way that thevalues of the control parameters are statistically distributed in such away that a mean value corresponds to a preset desired value for thecontrol parameter.

A closed-loop control system can be provided to carry out a leakcompensation.

In particular, it is intended that the leak compensation is carried outdynamically.

To further support ventilation, it is proposed that a determination ofthe spontaneous respiratory behavior be performed by the evaluationunit.

In particular, it was found to be advantageous for the evaluation unitto compensate the effect of spontaneous respiratory behavior on theventilation.

In a typical embodiment, leak detection is carried out in an areabetween a ventilator and a patient.

In accordance with another modification, it is proposed that themeasurements be carried out only during inspiratory phases of therespiratory cycles.

To detect the ventilation pressure, it is proposed that at least onepressure sensor be connected to the evaluation unit.

Ventilation flow can be detected by connecting at least one volume flowsensor to the evaluation unit.

The arrangement of at least one of the sensors so that it faces aventilator that is supplying the respiratory gas is conducive to acompact design of the device.

Improved measuring accuracy can be obtained by arranging one of thesensors so that it faces a ventilation mask.

In a typical embodiment, an expiration valve is arranged so that itfaces the ventilation mask.

The arrangement of a discharge system so that it faces the ventilatoralso contributes to a compact design of the device.

In clinical applications, it is possible for a patient interface that isconnected with the ventilator by the respiratory gas hose to be designedas an invasive device.

In particular, however, it is intended that a patient interface that isconnected with the ventilator by the respiratory gas hose be designed asa noninvasive device.

In one embodiment of a signal generator, the evaluation unit has anamplitude generator for a pressure that varies from respiratory cycle torespiratory cycle.

In accordance with another embodiment, it is provided that theevaluation unit has an amplitude generator for a volume flow that variesfrom respiratory cycle to respiratory cycle.

Specific embodiments of the invention are shown schematically in thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a functional block diagram of the device illustrating itsessential components.

FIG. 2 shows an electrical circuit diagram analogous to the lung of apatient, which is considered as a model in the control of the device.

FIG. 3 shows a graph that illustrates the differences of measuredquantities between two inspirations.

FIG. 4 shows a graph for the reconstruction of the course with respectto time of spontaneous respiration with evaluation of the differencevalues with respect to pressure and volume flow during two inspirationswith different inspiratory volume flows.

FIG. 5 shows a functional block diagram for illustrating the design ofthe device.

FIG. 6 shows a functional block diagram of a modified design of thedevice.

FIG. 7 shows another modification of the design of the device.

FIG. 8 shows a functional block diagram of another modified design ofthe device.

FIG. 9 shows another modification of the device.

FIG. 10 shows another modification.

FIG. 11 shows another modification.

FIG. 12 shows another modification.

FIG. 13 shows another modified embodiment.

FIG. 14 shows another modified embodiment.

FIG. 15 shows an equivalent circuit diagram that has been modified fromthe diagram in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment shown in FIG. 1, a ventilation system 1 has a drivenfan 3. The fan 3 is driven by a motor or some other power source. Thefan 3 is connected to a control valve 4, which has a control unit 6 andtransforms a control voltage to an associated valve position.

In the illustrated embodiment, a pressure sensor 8 and a volume flowsensor 9 are connected to a ventilator 7, which consists essentially ofthe fan 3 and the control valve 4. The volume flow sensor 9 is typicallydesigned as a differential pressure sensor, whose signal is converted toan associated volume flow.

The ventilator 7 is connected by a respiratory gas hose 10 and anexpiration valve 11 to a ventilation mask 12, which can be positionedover the face of a patient 13.

The sensors 8, 9 are connected to an evaluation unit 14, which in turnis connected to the control unit 6.

FIG. 2 shows an equivalent electrical circuit diagram, which reproducesthe function of the lung of the patient 13 and is used in the evaluationunit 14 as a model for performing the computations. A volume flowgenerated by a volume flow source 15 is supplied to the parallelconnection of two flow branches. One of the branches contains the seriesconnection of the resistance 16 and the compliance 17 and a source ofinterference 18, which generates an additional volume flow andrepresents possible activity of the patient 13 himself. The second flowbranch contains a leakage resistance 19.

The equivalent electrical circuit diagram of the respiratory tractaccording to FIG. 2 shows that the pressure P_(mus) of a patientproduced by his active breathing activity contributes to the airwaypressure P_(aw) and thus to the flow into the lung V′_(aw). Accordingly,leak identification must also take P_(mus) into account if the leakageflow V′_(L) is to be adequately compensated.

Assuming that the respiratory effort of the patient between inspirationsvaries by only an insignificant amount, the respiratory effort componentcan be eliminated by subtraction. The following relationship in thecomplex variable domain of the Laplace transform is obtained from theequivalent circuit diagram according to FIG. 2 for the airway pressureP_(aw) and the inspiratory flow V′_(insp):P _(aw)(s)=[P _(mus)(s)+V′ _(insp)(s)(1+sRC)/sC]×sR _(L) C/(1s(R+R_(L))C)

The differences of the airway pressure Δp_(aw) and the inspiratory flowΔV′_(insp) are computed as follows:ΔP _(aw)(s)=P _(aw,k)(s)−P _(aw,k1)(s)ΔV′ _(insp) (s)=V′ _(insp,k)(s)−V′ _(insp,k 1)(s)where k is the running number of the inspiration.

Assuming that the respiratory effort with respect to time is the same insuccessive inspirations, i.e., assuming thatP _(mus,k)(s)=P _(mus,k1)(s)a simpler relationship is obtained between the pressure differenceΔp_(aw) and the flow difference ΔV′_(insp) in the complex variabledomain of the Laplace transform:ΔP _(aw)(s)=ΔV′ _(insp)(s)(1+sRC)R _(L)/(1+s(R+R _(L))C)

To reconstruct the behavior of P_(mus), the inversion of the firstequation must be used. To this end, the identified estimated values areused as parameters, and the measured values are accounted for.P^ _(mus)(s)=(1+s(R^+R^ _(L))C^)/sR^ _(L) C^P _(FS)(s)−(1+sR^C^)/ sC^V′_(FS)(s)

A reconstruction of this type allows an evaluation of the spontaneousrespiration and thus indirectly an evaluation of the quality ofventilation. In the simplest case, it can be decided by integration ofthe reconstructed values whether the patient is breathing with oragainst. For the first time, more complicated methods of classificationcan derive from this a decided evaluation for the volume-controlledventilation, for example, with respect to synchronization or the levelof respiratory effort.

FIG. 3 shows the behavior with respect to time of the difference of themeasured quantities between two respiratory cycles, one being aninspiratory volume flow difference 20, and the other being a pressuredifference 21 in the area of the ventilation mask 12.

FIG. 4 shows, with respect to a first pressure curve 22 for a firstrespiratory cycle and a second pressure curve 23 for a secondrespiratory cycle, the determined reconstruction curve 24 forspontaneous respiration of the patient 13, taking into account thevolume flow difference 20 and the pressure difference 21 between twoinspirations with different inspiratory volume flows.

FIG. 5 shows the device illustrated in FIG. 1 in the form of afunctional block diagram. The ventilator 7 is connected to a respiratorygas hose 10, and the volume flow sensor 9 and the pressure sensor 8 arearranged in the area between the ventilator 7 and the respiratory gashose 10. The respiratory gas hose 10 is connected to the ventilationmask 12, and an additional volume flow sensor 25 and an additionalpressure sensor 26 are positioned in the area between the respiratorygas hose 10 and the ventilation mask 12. As an alternative to theillustration in FIG. 5, the sensors can also be arranged only in thearea between the ventilator 7 and the respiratory gas hose 10 or only inthe area between the respiratory gas hose 10 and the ventilation mask12. Naturally, a setup with a mass flowmeter is also possible; and inthis case, a conversion to volume flow is made.

In accordance with the embodiment shown in FIG. 6, an expiration valve11 is arranged in the area between the respiratory gas hose 10 and theventilation mask 12, and a discharge system 27 is arranged between theventilator 7 and the respiratory gas hose 10. It is also possible toarrange only the expiration valve 11 in the area between the respiratorygas hose 10 and the ventilation mask 12 or to arrange only the dischargesystem 27 in the area between the ventilator 7 and the respiratory gashose 10. Furthermore, it is possible to replace the expiration valve 11with a discharge system 27 and the discharge system 27 with anexpiration valve 11.

As an alternative to the ventilation mask 12 shown in the specificembodiments, other noninvasive devices can be used to provide aconnection with the patient 13. For example, the use of masks orheadpieces is possible. A connection between the ventilation system 1and the patient 13 can also be established by invasive coupling devices,for example, an intubation tube, a tracheostoma, or a laryngeal mask.

The ventilation systems that are used can be designed for carrying outvarious types of ventilation, for example, control mode ventilation,assist/control mode ventilation, or assist mode ventilation. Inaddition, the method can be used in periodic breathing and for CPAPventilation or APAP ventilation.

The determination of resistance and compliance can be made, for example,exclusively during the inspiratory or exclusively during the expiratorytime intervals of the respiratory cycles. However, it is also possibleto make these determinations during both the inspiratory and expiratoryphases.

FIG. 7 shows the previously mentioned embodiment, in which therespiratory gas hose 10 is directly coupled with the ventilation mask12, and the discharge system 27 and the sensors 8, 9 are positioned onlyin the area between the ventilator 7 and the respiratory gas hose 10.

FIG. 8 shows an embodiment that is a modification of the embodimentshown in FIG. 7, in which the discharge system 27 and the volume flowsensor 9 are positioned between the ventilator 7 and the respiratory gashose 10, and the pressure sensor 8 is positioned between the respiratorygas hose 10 and the ventilation mask 12.

In the embodiment in FIG. 9, the discharge system 27 and the pressuresensor 8 are positioned between the ventilator 7 and the respiratory gashose 10, and the volume flow sensor 9 is positioned between therespiratory gas hose 10 and the ventilation mask 12.

In the modification shown in FIG. 10, only the discharge system 27 isarranged between the ventilator 7 and the respiratory gas hose 10, andboth the volume flow sensor 9 and the pressure sensor 8 are positionedbetween the respiratory gas hose 10 and the ventilation mask 12.

In the modification shown in FIG. 11, both the pressure sensor 8 and thevolume flow sensor 9 are positioned between the ventilator 7 and therespiratory gas hose 10, and the expiration valve 11 is positionedbetween the respiratory gas hose 10 and the ventilation mask 12.

In the embodiment shown in FIG. 12, only the volume flow sensor 9 isarranged between the ventilator 7 and the respiratory gas hose 10, andthe expiration valve 11 and the pressure sensor 8 are positioned betweenthe respiratory gas hose 10 and the ventilation mask 12.

In the embodiment shown in FIG. 13, only the pressure sensor 8 islocated between the ventilator 7 and the respiratory gas hose 10, andboth the expiration valve 11 and the volume flow sensor 9 are locatedbetween the respiratory gas hose 10 and the ventilation mask 12.

Finally, FIG. 14 shows an embodiment in which the ventilator 7 isdirectly coupled with the respiratory gas hose 10, and the expirationvalve 11 and both the pressure sensor 8 and the volume flow sensor 9 arepositioned between the respiratory gas hose 10 and the ventilation mask12.

FIG. 15 shows an equivalent circuit diagram that is supplementedrelative to the diagram in FIG. 2 and is used in the case of linearbehavior of the system comprising the patient and the respiratory gashose. In an additional branch, the capacity 28 resulting from the volumeof the hose is taken into consideration.

For the relationship between the measured data for flow V′_(FS) andpressure p_(FS) in the device, the transfer function corresponding tothe following equation is obtained from the equivalent circuit diagramin the complex variable domain of the Laplace transform, neglecting aPEEP (positive end-expiratory pressure):P _(FS)(s)/V′ _(FS)(s)=(R _(L)×(1+sRC))/(1+s(RC+R _(L) C+R _(L) C_(s))+S ² RCR _(L) C _(s))

In this regard, the transit time behavior of the pressure inside thehose was disregarded (P_(FS)=P_(aw)), and R_(L) was adopted as the leakresistance.

After a time discretization of this equation with a zero-order hold(s−>(1−z⁻¹)/T), with the discrete-time shift operator z⁻¹, and with thesampling time T, the following representation of the relationshipbetween the sequences of values P_(FS, k) and V′_(FS, k) is obtained:P _(FS,k) /V′ _(FS,k)=(b ₀ +b ₁ Z ⁻¹)/(1+a ₁ Z ¹ +a ₂ Z ²)where

b₀=R_(L)T(T+RC)/K

b₁=−RCR_(L)T/K

a₁=(−T(RC+R_(L)C+R_(L)C_(s))−2RCR_(L)C_(s))/K

a₂=RCR_(L)C_(s)/K

K=T²+T(RC+R_(L)C+R_(L)C_(s))+RCR_(L)C_(s)

If we now consider the conditions on the patient side of the respiratorygas hose, the following transfer function between V′_(insp) and P_(aw)is obtained:P _(aw)(s)/V′ _(insp)(s)=R _(L)×(1+sRC)/(1+s(R+R _(L))+C)P _(aw,k) /V′ _(insp,k)=(b ₀ +b ₁ z ¹)/(1+a ₁ z ¹)where

b₀=R_(L)(T+RC)/L

b₁=−R_(L)RC/L

a₁=−(R+R_(L))C/L

L=T+(R+R_(L))C

If the transit time of the air pressure through the respiratory gas hoseis taken into account, then P_(FS) is not equal to P_(aw). For pressuresensors on the device side, the expansion withP _(FS)(s)=P _(aw)(s)/(1+sT _(s) ×e ^(sTt))can be used, in which T_(s) is the integral-action time of the flexiblerespiratory gas hose with the hose connector. The delay time Ttcorresponds to the sound transit time through the hose.

A higher degree of accuracy of the models in the above equations can beused to take into account such effects as gas compressibility. Moreprecise formulations can also be used in the transition to thediscrete-time representation.

The parameters R, C, and R_(L) can be estimated by well-knownidentification and parameter estimation methods for linear as well asnonlinear systems. Especially a time-distributed computation of theparameter estimation is advantageous for implementation. In this regard,recursive methods make it possible to obtain results with the greatesttime proximity to the last observation period.

1. Method for detecting leaks in respiratory gas supply systems, inwhich both the pressure of the respiratory gas and the volume flow ofthe respiratory gas are detected and fed into an evaluation unit,wherein the respiratory quantities pressure and flow are recorded by theevaluation unit for at least two successive respiratory cycles, that atleast one control parameter with different signal amplitudes is presetfor successive respiratory cycles, and that resistance, compliance, andleak resistance are determined by a computation based on differentialcurves of pressure and flow for these respiratory cycles.
 2. Method inaccordance with claim 1, wherein the computation is performed for atleast two immediately successive respiratory cycles.
 3. Method inaccordance with claim 1, wherein the computation is performed for atleast two respiratory cycles that are separated by at least one otherrespiratory cycle.
 4. Method in accordance with claim 1, wherein firstand second pressure levels are preset for the successive inspirations.5. Method in accordance with claim 4, wherein the first pressure levelis selected higher than the second pressure level.
 6. Method inaccordance with claim 4, wherein the first pressure level is selectedlower than the second pressure level.
 7. Method in accordance with claim1, wherein first and second volume flows are preset for the successiverespiratory cycles.
 8. Method in accordance with claim 7, wherein thefirst volume flow is preset higher than the second volume flow. 9.Method in accordance with claim 7, wherein the first volume flow ispreset lower than the second volume flow.
 10. Method in accordance withclaim 1, wherein a large number of respiratory cycles, each with avaried control parameter, are carried out in such a way that the valuesof the control parameters are statistically distributed in such a waythat a mean value corresponds to a preset desired value for the controlparameter.
 11. Method in accordance with claim 1, wherein a leakcompensation is carried out.
 12. Method in accordance with claim 11,wherein the leak compensation is carried out dynamically.
 13. Method inaccordance with claim 1, wherein a determination of spontaneousrespiratory behavior is performed by the evaluation unit (14). 14.Method in accordance with claim 1, wherein the evaluation unit (14)compensates the effects of spontaneous respiratory behavior.
 15. Methodin accordance with claim 1, wherein leak detection is carried out in anarea between a ventilator (7) and a patient (13).
 16. Method inaccordance with claim 1, wherein measurements are carried out onlyduring inspiratory phases of the respiratory cycles.
 17. Device fordetecting leaks in respiratory gas supply systems, comprising a devicefor detecting the pressure of a respiratory gas and a device fordetecting the volume flow of the respiratory gas, wherein the detectiondevices are connected to an evaluation unit, wherein the evaluation unit(14) is configured to determine the respiratory quantities pressure andflow, further comprising a storage device for at least one pair of valuesequences of pressure and flow for a respiratory cycle, and means forgenerating at least one differential sequence for determiningdifferential curves of compliance and resistance for at least twosuccessive respiratory cycles.
 18. Device in accordance with claim 17,wherein at least one pressure sensor (8) is connected to the evaluationunit (14).
 19. Device in accordance with claim 17, wherein at least onevolume flow sensor (9) is connected to the evaluation unit (14). 20.Device in accordance with claim 17, wherein at least one of the sensors(7, 8) is arranged so that it faces a ventilator (7) that is supplyingthe respiratory gas.
 21. Device in accordance with claim 17, wherein oneof the sensors (8, 9) is arranged so that it faces a ventilation mask(12).
 22. Device in accordance with claim 17, wherein an expirationvalve (11) is arranged so that it faces the ventilation mask (12). 23.Device in accordance with claim 17, wherein a discharge system (27) isarranged so that it faces the ventilation mask (12).
 24. Device inaccordance with claim 17, wherein a discharge system (27) is arranged sothat it faces the ventilator (7).
 25. Device in accordance with claim17, wherein an expiration valve (11) is arranged so that it faces theventilator (7).
 26. Device in accordance with claim 17, wherein apatient interface that is connected with the ventilator (7) by therespiratory gas hose (10) is designed as an invasive device.
 27. Devicein accordance with claim 17, wherein a patient interface that isconnected with the ventilator (7) by the respiratory gas hose (10) is anoninvasive device.
 28. Device in accordance with claim 17, wherein theevaluation unit (14) has an amplitude generator for a pressure thatvaries from respiratory cycle to respiratory cycle.
 29. Device inaccordance with claim 17, wherein the evaluation unit (14) has anamplitude generator for a volume flow that varies from respiratory cycleto respiratory cycle.